The primary objective of this proposal is to investigate the in vitro proliferation of retinal vascular endothelial cells in response to platelet factor and other mitogens. An in vitro assay system will be used in which thymidine incorporation by endothelial cells is quantitated after mitogenic stimulation. Both established and newly available techniques will be used to provide the requisite homogenous endothelial cell cultures. These will include homogenization of bovine retinas, antibody labelling of endothelial cells derived from capillaries isolated in vitro, and electronic cell sorting of these labelled cells from the mixed cell population. This investigation is of particular importance since the in vitro behavior of retinal vascular endothelium has not been characterized to date. Unlike endothelia from other sites, these morphologically and functionally unique cells may react quite differently to known growth factors. Achievement of the objectives may someday lead to an in vitro model of the in vivo vascular proliferation produced by blinding systemic diseases.